1. Field of the Invention
This invention relates to pneumatic tires for two-wheeled vehicles and more particularly to a pneumatic tire for motorcycles which can improve high speed running stability.
2. Description of the Prior Art
Motorcycles (hereinafter called two-wheeled vehicles) are far more influenced by exterior disturbance due to undulation or local inclination of the road surface, side wind and non-uniformity of the tire or the like when the vehicles run when compared with four-wheeled vehicles. Particularly, if the two-wheeled vehicle running straight ahead at a high speed is subjected to the exterior disturbance in the horizontal direction, the vehicle body becomes severely oscillated. Particularly, the exterior disturbance subjected to the front wheel induces a conspicuous handle bar oscillation so that a driver feels apprehensive, and eventually cannot continue the vehicle.
The inventor's investigations have demonstrated the result that even if a new tire including a tread rubber whose gauge is sufficiently large is subjected to a force changeable in the horizontal direction and causing the above mentioned vehicles's oscillation, almost all such force is absorbed by the tread rubber and hence the vehicle's oscillation is not induced. On the contrary, as the tread rubber becomes worn the remaining tread rubber loses its oscillation absorbing ability, the force changeable in the horizontal direction is transmitted through the tire carcass and wheel to the vehicle body. In addition, it has been ascertained that the above mentioned phenomenon is accelerated when use is made of a carcass ply having a high strength for the purpose of reducing the weight of tire and making the tire suitable for a high speed running and hence the number of the carcass plies is reduced.